This invention relates generally as indicated to an aircraft supplemental air heater and, more particularly, to an air heater wherein a composite containing the heating element has a coiled construction and a spiral-shaped air passage is provided for convective heat transfer.
An aircraft, such as a commercial plane, conventionally has a heating system for climate control of cabin air. This heating system comprises, for example, a heat exchanger which uses warm engine exhaust gas to heat cabin air and this heated air is then supplied to the cabin through a series of air ducts. While such a heating system usually provides sufficient heat for a majority of areas on the aircraft, there are usually still some xe2x80x9ccold areasxe2x80x9d which require supplemental heat. These cold areas include, for example, lavatories, certain passenger seats (such as the ones adjacent emergency exit doors), the cockpit, the cargo bay, and other areas. Supplemental air heaters may be required on the aircraft to maintain desired thermal conditions for the aircraft""s crew, passengers and/or equipment in these cold areas. Typically, a supplemental aircraft air heater is installed in-line with a cabin air supply duct and/or downstream of a fan providing a forced airstream.
An aircraft supplemental air heater may comprise an outer shell and a heating element within the shell. Typically, the heating element is insulated and bonded to one side of a flat carrier plate or contained within a carrier plate. The electrical heating element provides a heat output when a voltage potential is applied across it and the heat output is transferred to the flat carrier plate. The carrier plate is positioned within the shell in an orientation parallel to the airflow direction so that as the cabin air flows through the heater""s shell, it passes over the heater plate and heat is thereby convectively transferred to the air.
An aircraft supplemental air heater having a flat carrier plate will usually perform sufficiently well in many situations, however, some issues are created in circumstances requiring higher heat output. Specifically, to satisfy higher heat output demands, the aircraft supplemental air heater may need to incorporate a plurality of heater plates to increase the convective heat transfer surface. When a plurality of heater plates are used, they are usually placed in parallel or in series with each other. In any event, the introduction of multiple heater plates usually increases production costs and complicates assembly, installation and wiring procedures. Additionally or alternatively, the increased power density required by the multiple heater plates elevates operating temperatures and reduces heater life.
The present invention provides a lightweight supplemental air heater for an aircraft. More particularly, the present invention provides an aircraft supplemental air heater comprising a coiled composite defining a spiral shaped airflow passage. The coiled composite includes a heating element that is configured to operate at different voltages and/or may be configured to operate in multi-phase AC systems. If the heating element comprises an etched metal layer, it may comprise a plurality of different heating circuits. In any event, the heating element provides a controlled heat output when a voltage potential is applied across it. The composite also includes a convective heat transfer surface to which the heat output is transferred whereby as air flows through the heater""s spiral shaped airflow passage, it passes over the convective heat transfer surface and heat is thereby transferred to the air through convection.
An aircraft supplemental air heater according to the present invention may be constructed so that the radial dimension of the coil is between 0.2xe2x80x3 and 1.0xe2x80x3. The spiral-shaped passage may comprise a central substantially cylindrical passageway, coil-shaped passageways between radially adjacent coil passes and an outer passageway surrounding an outer coil pass. The separation in the radial direction between adjacent coil passes may remain substantially constant whereby the radial dimension of the coil-shaped passageways will be approximately the same along the radial dimension of the coiled composite. Alternatively, the separation in the radial direction between adjacent coil passes may change between coil passes whereby the radial dimension of the coil-shaped passageways will vary along the radial dimension of the composite.
The aircraft supplemental air heater may be installed in-line with an air supply duct leading to the cabin of the aircraft. If so, the coiled composite may be positioned approximately concentrically within a shell having a size and shape corresponding to the air supply duct. Additionally or alternatively, the aircraft supplemental air heater may be installed downstream of a fan. If the fan creates an airflow profile in which the fluid velocity decreases in the radially inward direction, the heater may be constructed so that the radial separation between adjacent coil passes is greater at radially center passageways than radially outer passageways. In this manner, slower moving portions of the airstream""s velocity profile will be provided with wider passageways through the heater. Alternatively, the density of the heating element(s) may be varied in relation to the airstream""s velocity profile.
To make an aircraft supplemental air heater according to the present invention, a flat composite panel is assembled and then this panel is formed into the desired coiled shape. The panel initially has a length corresponding to the desired length of the element, a width corresponding to the desired number of coil passes and the desired radial dimensions of the air passageways, and a thickness corresponding to the desired radial dimension of the coil passes.
The composite panel may be compiled from a metal foil layer which is processed to form the heating element, insulation layers which form the insulation strata, and a carrier layer which forms the carrier sheet. In this compilation, for example, the metal foil layer is bonded to one of the insulating layers and the desired conductive paths are etched in the metal foil layer. The etched foil layer is then covered with the other insulating layer and the carrier sheet is secured thereto to form the composite panel.
The panel may be formed into the coiled shape by placing a spacer on the composite panel, winding the panel/spacer into a coiled shape, curing the coiled panel/spacer so that it maintains the coiled shape, and removing the spacer. The spacer has a thickness corresponding to the desired separation between the coil passes in the completed heater. If the spacer comprises a spacer layer, this layer may have substantially the same thickness across its width/length to provide substantially the same radial dimension between the coil passes or this layer may have a varying thickness to provide differential radial dimensions between the coil passes. The spacer may also comprise a spacer element which is shaped and sized to correspond to the desired central passageway of the heater.
The coiled construction of the composite and other features of the air heater provides advantages over conventional air heaters which often must include several carrier plates to satisfy higher heat output requirements. In comparison to these multi-plate heaters, the heater according to the present invention is economically produced, quickly assembled, efficiently installed, easily wired, and provides increased reliability. As a result of some or all of these advantages, local power density can be reduced thereby lowering operating temperatures and prolonging heater life. Also, the heater of the present invention may be constructed so that its air passage compensates for a fan-induced flow profile, a compensation that is not so easily accomplished with conventional air heaters having multiple carrier plates.
These and other features of the invention are fully described and particularly pointed out in the claims. The following descriptive annexed drawings set forth in detail certain illustrative embodiments of the invention, these embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed.